In exhaust gas discharged from a lean burn engine such as a boiler, a gas turbine, a lean burn-type gasoline engine, or a diesel engine, various harmful substances derived from fuel or combustion air are included. Such harmful substances include a hydrocarbon (HC), a soluble organic fraction (it may also be called SOF), soot, carbon monoxide (CO), nitrogen oxides (NOx) and the like. Regulations on discharge amount of these harmful components have been strengthening year by year, and as a purification method of these harmful components, there has been practically used a method for purifying exhaust gas by making it contacted with a catalyst.
In such a lean burn engine, there has also been investigated on suppression of generation amount of harmful substances by controlling kinds of fuel, supply amount, and supply timing of fuel, amount of air or the like. However, satisfactory purification of exhaust gas has not been attained by a conventional catalyst or a control method. In particular, in a lean burn engine, nitrogen oxides are easily discharged and regulation thereof has been strengthening more and more, however, in the case of a diesel engine loaded on an automobile, it is difficult to suppress discharge of the harmful substances by conventional NOx purification technology, because operation condition thereof is always changing.
Further, in recent year, regulations of discharge amount of carbon dioxide (CO2), which is the greenhouse effect gas, has been strengthened. Because discharge amount of CO2 is proportional to fuel amount used in engine operation, it has been desired that, in a combustion engine, used amount of fuel is small and has good fuel efficiency. A diesel engine is a combustion engine having good fuel efficiency and small discharge amount of CO2, however, includes a large quantity of NOx in exhaust gas.
To suppress discharge of NOx in a diesel engine, it is considered to make air/fuel ratio small, and supply to an engine a large quantity of fuel, which is also a reducing agent, however, it incurs deterioration of fuel efficiency, and also increases discharge of CO2. In addition, such a combustion control cannot utilize advantage of a diesel engine, that is, good fuel efficiency.
As technology for purification of NOx (hereafter it may be referred to as denitration, or De-NOx), there has been known technology for denitrating by reduction, where exhaust gas including NOx is contacted with a selective reduction catalyst having titanium oxide, vanadium oxide, zeolite and the like as main components, under presence of ammonia (NH3) component, as a selective reduction method or a Selective Catalytic Reduction (hereafter it may be referred to as SCR) method.
In the SCR, where this NH3 component is used as a reducing agent, NOx is finally reduced to N2 mainly by the following reaction formulas (1) to (3):4NO+4NH3+O2→4N2+6H2O  (1)6NO2+8NH3+O2→7N2+12H2O  (2)NO+NO2+2NH3→2N2+3H2O  (3)
In denitration in exhaust gas, it is theoretically enough that NH3/NOx molar ratio is 1.0, in the above denitration reaction formulas (1) to (3), however, in the case of transient engine operation condition in operation of a diesel engine, or in the case where space velocity, temperature of exhaust gas, or temperature of the catalyst surface is not suitable, there is the case where NH3/NOx ratio of the NH3 component to be supplied to obtain sufficient denitration performance is inevitably increased, resulting in leakage of unreacted NH3, where a risk of incurring secondary pollution, such as new environmental contamination, has been pointed out. Hereafter, leaked NH3 may be referred to as slip, or NH3 slip.
In such a denitration catalyst system, NH3 gas may be used as the reducing component, however, because NH3 itself has irritating odor or hazardous property, there has been proposed a system for adding urea water, instead of NH3 gas, from the upstream of the denitration catalyst, generating NH3 by pyrolysis or hydrolysis, and having this acted as a reducing agent to exert denitration performance.
Reaction formulas for obtaining NH3 by decomposition of urea in this way are the following (4) to (6). Here, (4) is pyrolysis reaction of urea, (5) is hydrolysis of isocyanic acid and (6) is hydrolysis of urea.NH2—CO—NH2→NH3+HCNO  (4)HCNO+H2O→NH3+CO2  (5)NH2—CO—NH2+H2O→2NH3+CO2  (6)
Urea is spray supplied as urea water, from the upstream of the SCR catalyst. As described above, because a component contributing to reduction purification of NOx is mainly NH3, a reaction of NOx in the SCR catalyst is influenced by decomposition efficiency of urea. Low decomposition efficiency of urea naturally decreases NOx purification performance, as well as increases used amount of urea, and could induce NH3 slip by unreacted urea.
Against such NH3 slip, it was required to arrange an oxidation catalyst at the latter part of the SCR catalyst, so as to oxidize and purify slipped NH3. However, arrangement of such a catalyst for purification of slipped NH3 leads to cost increase, and it was difficult to secure a loading place of the catalyst, in particular, in an automobile.
In addition, increased amount of slipped NH3 required the catalyst to have high oxidation capability, which then required use of a large quantity of an expensive noble metal such as platinum, which is active species.
In addition, in NOx purification by the NH3 component, the reaction is accelerated under atmosphere including NO and NO2 each nearly half as in the above formula (3) (NON PATENT LITERATURE 1). However, most of NOx components discharged from a lean burn engine is nitrogen monoxide (PATENT LITERATURE 1). Therefore, in order to increase concentration of NO2 component in exhaust gas, so as to attain efficient purification of NOx, there has been proposed arranging an NO oxidation means at an exhaust gas passage (PATENT LITERATURE 2).
There has also been proposed a method for purifying harmful particulate components and NO in one catalyst system at the same time, by utilization of such a NO oxidation means. One of them is the one for arranging the oxidation catalyst in exhaust gas passage, arranging a filter at the latter part thereof, spraying the ammonia component at the latter part thereof, and arranging the SCR catalyst at the latter part thereof (PATENT LITERATURE 3).
In addition, purification technology of soot or SOF (they may hereafter be referred to collectively as a “particulate component” or PM: Particulate Matter) influences also on fuel efficiency enhancement of a diesel engine. As for the particulate component, there has practically been used a method for arranging a heat resistant filter (DPF: Diesel Particulate Filter) in exhaust gas passage, and filtering off the particulate component with this filter. The particulate component thus filtered off deposits on the filter, and continued deposition of the particulate component on the filter causes clogging of the filter thus incurring decrease in output of an engine. Accordingly, there has been investigated regeneration of the filter by combustion removal of the particulate component deposited on the filter (PATENT LITERATURE 3, PATENT LITERATURE 4).
In the system of PATENT LITERATURE 3 and PATENT LITERATURE 4, by arranging DPF at the latter part of DOC, the particulate component deposited on the filter is removed by combustion using NO2 in addition to oxygen. Use of NO2, because of starting combustion of the particulate component from low temperature, not only promotes combustion removal of the particulate component but also enables to prevent melting of the filter by decreasing combustion temperature. Among the filters for combustion removal by capturing the particulate component in this way, DPF covered with a catalyst component is also referred to as CSF (Catalyzed Soot Filter).
In addition, there has also been proposed a purification method for combustion removal of the particulate component at the same time as NOx purification (PATENT LITERATURE 2, PATENT LITERATURE 4). In these methods, there has been proposed such one where an oxidation catalyst, a filter for filtering off particulate components, a supply means of an ammonia component, and a selective-type reduction catalyst, are arranged in this order in an exhaust gas flow, or such one where an oxidation catalyst, a supply means of an ammonia component, a selective-type reduction catalyst, and a filter for filtering off particulate components are arranged in this order in an exhaust gas flow (PATENT LITERATURE 5, PATENT LITERATURE 6). And, also among these systems, there may be the case where CSF catalyzed DPF is used to promote combustion of the particulate component.
In such an arrangement, by oxidation of NO in exhaust gas to NO2 using the oxidation catalyst, combustion removal of the particulate component and reduction purification of NOx can be performed in one catalyst system at the same time. And, it has been known that a platinum component is effective as an oxidation catalyst component of this NO (PATENT LITERATURE 4, NON PATENT LITERATURE 4).
As such purification method for performing purification of NOx and combustion removal of the particulate component at the same time, there has been developed the FLENDS system of Nissan Diesel Co., Ltd., or Bluetech of Daimler AG or the like, which is for a diesel automobile application, and prevalence thereof is progressing. In addition, as the reducing component, there is an aqueous solution of urea, having a specified concentration of 31.8 to 33.3% by weight, now on the market as a trade name of “Adblue”.
In this way, a purification means for NOx and the particulate component has been proposed, however, in any of these cases, the object is to enhance efficiency of NOx purification in SCR by arranging DOC upstream of SCR, and increasing NO2 concentration in exhaust gas.
In addition, in recent years, with strengthening regulation of exhaust gas, there has been increasing tendency of number of catalysts to be used in an exhaust gas purification system corresponding to exhaust gas from a lean burn engine. In particular, in for an automobile, which is a mobile internal combustion engine, there is a problem of limited loading space of an apparatus, or requirements to attain low fuel efficiency and high output. To satisfy these requirements, there are needs for weight reduction and compact sizing of a catalyst per one unit, as well as decrease in pressure drop. The above PATENT LITERATURE has not investigated these problems, and thus cannot be said practical as an exhaust gas purification catalyst.
In recent years, the exhaust gas purification system aiming at purifying NOx, using a reducing component such as an aqueous solution of urea, by increasing concentration of NO2 by an oxidation catalyst, has raised a new problem in view of fuel efficiency enhancement. That is a problem of decrease in reducing performance of NOx, caused by leaking out and adhering of the platinum component used in DOC and/or CSF, to SCR downstream.
In DOC, the noble metal component such as platinum (Pt) or palladium (Pd) is used aiming at oxidation removal of HC or CO in exhaust gas, or oxidation purification of soot or SOF in exhaust gas, respectively, however, DOC also has action of oxidation of NO in exhaust gas to NO2, as described above. Exhaust gas having increased amount of NO2 promotes reduction purification of NOx in SCR downstream, and combustion of the particulate component at DPF or CSF.
In addition, increase in temperature of exhaust gas using HC in exhaust gas at DOC is effective to promote combustion removal of the particulate component deposited onto DPF or CSF arranged downstream of DOC. Therefore, in an exhaust gas purification system of a diesel engine, there may be the case where HC components are combusted (oxidized) by supplying the HC components to DOC. As a means for using the HC components to increase temperature of exhaust gas in this way, there is a method for supplying relatively more amount of fuel to an engine and generating unburned HC and supply it to DOC; or a method for supplying fuel to DOC by direct spraying.
In addition, increase in temperature of exhaust gas using HC in exhaust gas at DOC is effective to promote combustion removal of the particulate component deposited onto DPF or CSF arranged at the backward of DOC. Therefore, in an exhaust gas purification system of a diesel engine, there may be the case where HC components are combusted (oxidized) by supplying the HC components to DOC. As a means for using the HC components to increase temperature of exhaust gas in this way, there is a method for supplying relatively more amount of fuel to an engine and generating unburned HC and supply it to DOC; or a method for supplying fuel to DOC by direct spraying.
As described above, a diesel engine is a combustion engine having good fuel efficiency and small discharge amount of CO2, however, use of fuel aiming at increasing temperature of exhaust gas deteriorates fuel efficiency, and increases discharge amount of CO2. However, in many cases, temperature of exhaust gas of a diesel engine is 400° C. or lower, which is too low temperature for combustion removal of the particulate component deposited onto DPF (hereafter it may be referred to as regeneration of DPF) using exhaust gas as it is, therefore, to promote combustion of the particulate component, in particular, the soot component, there may be the case of heating exhaust gas at 600° C. or higher (JP-A-2003-148141, paragraphs 0012 and the like). To regenerate DPF or CSF by efficiently combusting the particulate component deposited onto DPF or CSF, it is necessary to frequently repeat it every time when the particulate component deposits onto DPF or CSF, which incurs deterioration of fuel efficiency. In addition, in the case of supplying fuel for regeneration of DPF or CSF to the inside of a cylinder after ignition, fuel is mixed into engine oil by supplying fuel frequently, and engine oil is diluted (Oil Dilution). Generation of Oil Dilution decreases output of an engine, caused by decrease in lubrication function of engine oil, and increase in oil amount in the engine.
Accordingly, it may be considered to decrease number of times of combustion removal of the particulate component deposited onto DPF or CSF, so as not to decrease fuel efficiency as good as possible, while promoting combustion of the particulate component deposited onto DPF or CSF. Decrease in number of times of regeneration is capable of suppressing temperature increase in exhaust gas and decreasing amount of fuel, and thus preventing deterioration of fuel efficiency. However, decrease in number of times of regeneration of DPF or CSF causes deposition of a large quantity of the particulate component onto DPF or CSF, therefore, it becomes necessary to perform oxidation removal of a large quantity of the particulate component at high temperature, in regeneration of DPF or CSF.
In this way, combustion removal of the particulate component by making temperature higher as compared with conventional methods, makes possible to perform combustion removal of a large quantity of the particulate component at one time. However, promotion of heat generation of DOC using a large quantity of the HC components results in exposure of DOC in exhaust gas from a diesel engine at such a high temperature atmosphere as over 800° C., although in a short period of time. In this case, a diesel engine automobile, in many cases, runs a longer distance as compared with a general gasoline engine automobile, therefore repeating regeneration of DPF or CSF for considerable number of times, resulting in exposure at high temperature for a long period of time, which raises a new problem of volatilization of platinum in DOC. Similarly, also in CSF, the catalyst component of CSF is exposed under high temperature atmosphere for a long period of time, which raises a new problem of volatilization of platinum in the catalyst component of CSF, in combustion removal of the particulate component held inside.
Platinum is oxidized at high temperature, volatilizes and adheres to the surface of SCR arranged at the backward of DOC and CSF, thus decreasing reducing performance of the catalyst (NON PATENT LITERATURE 2, NON PATENT LITERATURE 3). And, it is said that influence of the platinum component volatilized in SCR arranged at the backward of such DOC and CSF, is particularly significant in the case of using zeolite as the SCR catalyst.
In the case where activity of the SCR catalyst is lowered caused by the platinum component volatilized in this way, it becomes necessary to increase supply amount of the reducing agent such as urea or the ammonia component. However, supply of a large quantity of urea or the ammonia component could incur slip of ammonia from the SCR catalyst.
Because the noble metal in the catalyst may take various states such as an oxide state, an alloy state, and a composite oxide state with other metals, it is not easy to make clear as to by what reason the noble metal in the catalyst volatilizes, however, as for the platinum component, it can be considered roughly as follows.
Originally, when platinum is in a metal state, because volatilization temperature thereof is as high as 2090° C. (temperature at which vapor pressure becomes 10−2 Torr) under inert gas atmosphere, it is a metal not to be easily oxidized, even by being processed at high temperature under coexistence with air. However, it has been known that a platinum atom at the metal surface is oxidized when exposed to a high temperature of 850° C. or higher under coexistence with oxygen or air, and converted to a platinum oxide (PtO2) molecule and gradually volatilizes (NON PATENT LITERATURE 5).
In a diesel engine, because fuel is supplied into a cylinder together with a large quantity of air and combusted, a large quantity of oxygen is also included in exhaust gas. In addition, although temperature of exhaust gas of a diesel engine is low, temperature of a catalyst bed of DOC becomes over 700° C. in supplying the HC components to DOC, and there may be the case where it reaches 900° C. sometimes, and thus the platinum component is oxidized and exists in an easy volatilization state.
Further, because the platinum component in the catalyst maintains large surface area by changing to small particle diameter, in order to enhance oxidation activity, the Pt component in DOC is in a state that oxidized components are easily increased, and thus volatilization of the Pt component in the catalyst is worried.
As for suppressing such volatilization of the noble metal component at high temperature, there has already been investigated in TWC (Three Way Catalyst) for purification of HC, CO, and NOx in exhaust gas discharged from a gasoline engine at the same time (refer to PATENT LITERATURE 7). In this technology, a porous carrier is immersed in a solution of the noble metal to support the catalyst noble metal, and the noble metal supported carrier is immersed in a solution of an organic substance, and then this organic substance supported carrier is heat treated under condition for carbonization of the organic substance to suppress transfer of the Pt component into a vapor phase.
This conventional technology is the one for suppressing transfer of the Pt component into a vapor phase by the following three effects: anchor effect wherein heat treatment is performed under condition where the organic substance in a catalytic raw material is carbonized, and then the carbonized carbon enter into gaps between a porous carrier and the catalytic noble metal, and thus suppress transfer of the catalytic noble metal as a wedge; effect wherein the catalytic noble metal is three-dimensionally immobilized in the porous substance by making pores of the porous carrier shrink by heat treatment at a high temperature of 700° C. or higher; and effect for suppressing transfer of the catalytic noble metal utilizing a base metal such as Fe, Ni, or Co having superior heat resistance, as an obstacle.
However, this conventional technology is extremely difficult to leave carbon components required as ember, in production step of the catalyst, and thus cannot be said practical. In addition, even if the carbon components is left as ember in production, it is easily combusted when contacted with exhaust gas of high temperature in usage of the catalyst, thus maintaining of the effect for a long period of time cannot be expected. In addition, shrinkage of a porous substance by calcining decreases specific surface area (BET value) of the porous carrier, resulting in deterioration of dispersibility of the noble metal component and decrease in activity of the catalyst. In addition, a base metal such as Fe, Ni, or Co is a promoter component, and is not a component to be used necessarily in all catalysts in view of catalyst designing, in particular, Ni and Co are components envisaged to provide health hazard, therefore it is not preferable to use as an automotive catalyst.
In addition, purification of NOx in TWC is performed by the following steam reforming reaction, using a rhodium (Rh) component in the catalyst and HC in exhaust gas. And, use of a zirconium oxide together with the Rh component promotes the steam reforming reaction (JP-A-2000/027508, page 14).HC+H2O--------→COx+H2  (7)H2+NOx--------→N2+H2O  (8)
In NOx purification in exhaust gas of a gasoline engine in such TWC, and in NOx purification for processing exhaust gas of a diesel engine with the ammonia component of the reducing agent and the SCR catalyst, reaction steps thereof are basically different. Accordingly, catalyst technology in TWC cannot always be used as it is, as NOx purification technology for a diesel engine.
In addition, in order to suppress volatilization of the noble metal component from DOC, it may be considered not to use the Pt component as the noble metal component. However, non-use of the Pt component decreases NO2 concentration in exhaust gas, and there may be the case where sufficient reduction purification of NOx in SCR is not obtained, and decrease in NO2 concentration also deteriorates regeneration efficiency of DPF and CSF.
In addition, as a method for excluding decrease in SCR performance caused by the noble metal component which volatilizes from DOC or CSF, it may be considered to use the SCR catalyst component which has durability against contamination of the noble metal, and is capable of maintaining high NOx purification performance, for example, a vanadium oxide, as a main component. However, vanadium is a harmful heavy metal, and thus is not desirable in an automotive application.
In SCR, various kinds of zeolite have widely been used, however, zeolite significantly decreases SCR performance caused by contamination with the noble metal.
In this way, it has been desired a practical catalyst apparatus, not incurring decrease in NOx performance even at high temperature, in the catalyst apparatus arranged with SCR at the backward of DOC or CSF, in exhaust gas flow of a lean burn engine represented by a diesel engine.